A conventional single-channel virtual antenna array system makes use of wideband radio frequency (RF) signals and a large synthetic aperture to generate two-dimensional (2D) range-azimuth images. The 2D azimuth images, without any elevation information, are a projection of the 3D scene onto the 2D range-azimuth plane. Therefore, the 3D structure of the scene, such as a 3D terrain, is not preserved after the projection. In addition, this projection may cause several artifacts, such as layover and shadowing. In layover artifacts, several terrain patches with different elevation angles are mapped into the same range-azimuth cell. In shadowing artifacts, certain areas are not visible by the array imaging system because of occluding structures. These artifacts cannot be resolved by a single baseline observation, even using interferometric array imaging techniques.
In order to perform 3D imaging, multi-baseline observations are necessary in the elevation dimension. The multi-baseline observations can be acquired either by multiple passes of a single-channel platform or a single pass of a multiple-channel platform.
A moving MIMO system, as a multiple-channel platform with 3D imaging capability, has the following advantages. First, the degrees of freedom are greatly increased by the multiple antennas of the MIMO array. Second, the moving MIMO platform can provide much more transmitter-receiver combinations to satisfy cross-track sampling, resulting significantly improved elevation resolution.
However, the moving MIMO array platform also suffers from several tradeoffs. First, the total number of simultaneous transmitting channels are restricted to avoid self interference. For conventional MIMO array, the transmitting elements are typically fixed. Second, the spatial location of the moving MIMO array are subject to motion errors. This can cause ambiguity and defocus when left uncompensated.
As shown in FIG. 1, a conventional MIMO array system 110, which generally moves 120 along an azimuth (y) direction, generates a 3D image of a scene with point scatterers 130 at different elevations. The magnitude of the velocity vector is constant, and the direction is in a straight line. The array includes fixed receivers (x, ♦) and fixed transmitters (x) typically at each end of the array.
FIG. 2 shows a conventional 3D imaging method. Here, the MIMO array is moving at a constant velocity, and the transmitters and receivers are fixed. That is, all transmitters and receivers of the array are the same while transmitting and receiving. The transmitters emit radio frequency (RF) signal onto a scene, which are reflected and received by the receivers. The data 211 corresponding to the received RF signals 210 are used to generate 220 2D images 221 that are aligned 230. Then, 3D image reconstruction is applied to the aligned image to obtain the 3D image 241.